The recent drive for higher integration and operating speeds in LSI devices makes it necessary to further reduce the pattern rule. Deep-ultraviolet lithography was developed as an essential technology for micropatterning to a feature size of 0.3 μm or less. Among others, the KrF excimer laser lithography has been fully recognized as a commercial scale production technology.
With respect to chemically amplified resist compositions adapted for the photolithography using ArF excimer laser light of 193 nm wavelength as a light source, the primary requirement is, of course, a high transparency at that wavelength. They are also required to meet a high etch resistance sufficient to comply with film thickness reduction, a high sensitivity sufficient to minimize the burden to expensive optical materials, and among others, a high resolution sufficient to form an exact fine pattern. The key toward these requirements is to develop a base resin featuring high transparency, high rigidity and high reactivity. Active efforts have been devoted for such development.
Typical resins known to be highly transparent to ArF excimer laser light are copolymers of acrylic or methacrylic acid derivatives as disclosed in JP-A 4-39665.
One of the (meth)acrylic resins proposed thus far is a combination of (meth)acrylic units having methyladamantane ester as acid labile group units with (meth)acrylic units having lactone ring ester as adhesive group units as disclosed in JP-A 9-90637. Acid labile groups of exo form are described in U.S. Pat. No. 6,448,420 (JP-A 2000-327633). These groups have so high an acid elimination ability and require a low level of activation energy for acid elimination, affording a high resolution and low dependence on post-exposure bake (PEB). Norbornane lactone is also proposed as an adhesive group having enhanced etching resistance as disclosed in JP-A 2000-26446 and JP-A 2000-159758. These studies have achieved significant improvements in the resolution of ArF resists.
However, in an attempt to form a fine pattern having a pitch of less than 200 nm, prior art resist materials are difficult to form patterns and the patterns, if formed, have insufficient rectangularity and substantial roughness on their surface and sidewalls, and are hardly believed to clear the practically acceptable level. Of the problems associated with prior art resist materials, the most serious problem is the unevenness of fine line size, which is generally referred to as “line edge roughness” (LER). Since the LER has a substantial impact on the performance of semiconductor devices being fabricated, it is strongly desired to overcome this problem. Introducing many acid labile units is advantageous for achieving a higher sensitivity and higher contrast, but entails an enlargement of LER. An approach of reducing the amount of acid labile groups introduced and instead, introducing carboxylic acid units such as (meth)acrylic acid is successful in reducing LER to a certain extent while maintaining a resolution. The introduction of carboxylic acid units, however, gives rise to a new problem of surface roughening due to swelling. This approach does not lead to an essential reduction of LER.